1. Field of the Invention
This invention relates to photoconductors which have a photoconductive layer composed predominantly of amorphous silicon hydride and can be effectively utilized as one-dimensional or two-dimensional image sensors or electrophotographic photosensitive materials.
2. Description of the Prior Art
Photoconductors, which have an amorphous silicon hydride layer on a conductive substrate, have been recently developed and are now being studied because of the absence of public nuisance, their high photosensitivity, good resistance to heat and high spectrophotosensitivity over a whole range of visible light. These photoconductors have wide utility of the field of image sensors such as pickup tube targets and solid image pickup elements, or electrophotographic photosensitive materials.
For the formation of the photoconductive layer of amorphous silicon hydride on a substrate, several methods, are known including a plasma CVD method in which silane gas is decomposed in plasma, a reactive sputtering method in which silicon is provided as a target and hydrogen gas is introduced into the sputtering atmosphere along with an inert gas, and an ion plating method in which silicon vapor is reacted with hydrogen in hydrogen plasma.
The photoconductors, which are obtained by the plasma CVD and ion-plating methods, have the drawback that the resulting photoconductive layers are so small in resistance that the dark current becomes too large for use as image sensors or electrophotographic photosensitive materials. To avoid this, a blocking layer is provided between the photoconductive layer and the conductive substrate, or oxygen or nitrogen is introduced into the photoconductive layer in order to make a high resistant layer when the photoconductive layer of amorphous silicon hydride is formed by the plasma CVD method. Alternatively, as taught in Japanese Laid-open Patent Application No. 56-16434, when a small amount of selenium is added to amorphous silicon hydride on performing the plasma CVD method, the dark specific resistance increases up to about 10.sup.11 ohm-cm. However, this dark specific resistance value is not sufficient for image devices such as electrophotographic photosensitive materials.
In contrast, when the reactive sputtering method is used to form an amorphous silicon hydride layer, the resulting photoconductive layer has a resistance higher than the amorphous silicon hydride layer formed by the plasma CVD method. Although such a layer would be expected to be useful as an electrophotographic photosensitive material, it is disadvantageous in view of its poor photosensitivity. Accordingly, the photoconductive layer formed by the reactive sputtering method has not been studied actively.
We proposed formation of a photoconductive layer by the reactive sputtering method in which a chalcogen element was added to the amorphous silicon hydride layer in small amounts. This type of photoconductive layer had a specific resistance higher by about two orders of magnitude than the photoconductive layer of amorphous silicon hydride formed by the plasma CVD method, i.e. the specific resistance was about 10.sup.13 ohm-cm. Although the amorphous silicon hydride layer formed by the ordinary reactive sputtering method was disadvantageously low in photosensitivity, the chalcogen element-containing layer had a photosensitivity substantially equal to the photosensitivity attained by the plasma CVD method. For instance, when selenium was introduced into the photoconductive layer by the reactive sputtering method, the dark specific resistance slightly lowered with a specific resistance of 10.sup.13 ohm-cm. Thus, the layer had a high enough resistance for use in image devices and had high photosensitivity. However, in order to further improve the photoconductors, it is necessary to impart a high withstand-voltage to the photoconductor for use in image devices while keeping the high photosensitivity.
If the withstand-voltage is low, it is necessary to make the photosensitive layer thick when used as an electrophotographic photosensitive material so that the surface potential necessary for the electrophotographic process is ensured. This will present a problem from the standpoint of production cost. The photoconductive layer having a low withstand voltage is also disadvantageous in that white defects are liable to form upon application of corona voltage. Accordingly, further studies have been made to provide amorphous silicon hydride-base photoconductors which have a withstandvoltage as high as possible.